Method for determining stresses in soil

ABSTRACT

A novelly improved method of more economically expeditiously and more accurately obtaining stress determinations existing originally as in an undisturbed test soil, rock or other medium which method comprises cutting a smooth, very thin slot in the surface of the medium to be tested so as not to unduly disturb the stabilized natural structure of it and the adjoining medium, inserting a flat-type jack of a size and thinness corresponding essentially to the slot dimension so as to substantially completely fill the slot without need of infilling between the jack sides and test medium to thereby minimally and negligibly disturb the state of stress therein, subsequently applying on the walls of the major dimension of the slot, by means of a fluid pressure directed into said flat-jack, a force to cancel those minimal stresses in the test medium resulting from the initial cutting of the slot, and then finally measuring the pressure strains exerted by the fluid pressure in said jack on the sidewalls of the slot, whereby initial stress which existed in the test medium before the application of a disturbing action can be readily calculated. Such calculations may be derived from using the known mathematical theory of elasticity, and further a graphic curve can be plotted of the deformations as a function of the slot depth from which to derive the modulus of elasticity of the test medium.

United States Patent [72] Inventors Manuel Coelho Mendes Da Rocha Av"dos Estados Unidos da America, 95-3 Dt Jorge Neves Da Silva, Rua CaboVerde, 14- 2 Dt Joao Jose Andrade Baptista Lopes, Av do Brasil l82-I A,all of Lisbon,

Portugal [21 I Appl. No. 670,699 [22] Filed Sept. 26, I967 (45] PatentedJuly27,197l

[54] METHOD FOR DETERMINING STRESSES IN SOIL OTHER REFERENCESEngineering News-Record, June 7, 1934 Accuracy of Goldbeck Cell inLaboratory Tests" by W. H. Seaquist pp. 730 732 Primary Examiner-JerryW. Myracle Attorney-Wenderoth, Lind & Ponack ABSTRACT: A novellyimproved method of more economically expeditiously and more accuratelyobtaining stress determinations existing originally as in an undisturbedtest soil, rock or other medium which method comprises cutting a smooth,very thin slot in the surface of the medium to be tested so as not tounduly disturb the stabilized natural structure of it and the adjoiningmedium, inserting a flat-type jack of a size and thinness correspondingessentially to the slot dimension so as to substantially completely fillthe slot without need of infilling between the jack sides and testmedium to thereby minimally and negligibly disturb the state of stresstherein, subsequently applying on the walls of the major dimension ofthe slot, by means of a fluid pressure directed into said flat-jack, aforce to cancel those minimal stresses in the test medium resulting fromthe initial cutting of the slot, and then finally measuring the pressurestrains exerted by the fluid pressure in said jack on the sidewalls ofthe slot, whereby initial stress which existed in the test medium beforethe application of a disturbing action can be readily calculated. Suchcalculations may be derived from using the known mathematical theory ofelasticity, and further a graphic curve can be plotted of thedeformations as a function of the slot depth from which to derive themodulus ofelasticity of the test medium.

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METHOD FOR DETERMINING STRESSES IN SOIL The knowledge of the state ofstress inside the ground (soil, rock mass, etc.) is nowadays widelyrecognized as very important since from it it is possible to derive themechanical characteristics, bearing capacity, structure and so on andconsequently its behavior as the foundation of a structure.

Methods are now available by means of which stresses can be determinedin the ground both at the surface, especially in gallery walls, andinside at points reached by means of boreholes. The determination of thestate of stress from stress values measured in gallery walls has thedisadvantage that the initial state of stress is changed by theexcavation. This change is due, on one hand, to the presence of thehole, and on the other, to the excavation itself. Whereas the formerdisturbance can be taken into account by calculating the state of stressaround the gallery, it is otherwise with the latter. Hence the limitedinterest of a stress-determination method at the surface relative to thestate of stress inside, in spite of it being especially adequate forobtaining the field of stresses in the neighborhood of gallery walls,which is of the utmost interest for tunnel and gallery driving andmaintenance as tunnel-lining design and construction methods.

Among the methods in use for determining stresses at the surface, one ofthe most widely used is the so-called flat-jack method." As is wellknown, the method consists essentially in cutting a slot in the rock ortest soil medium with a given length and a certain depth, and inapplying on the slot walls flat-jacks filled with oil under pressurewhich transmit to the rock soil medium wall a uniform pressure whichcancels the deformations produced in the neighborhood of the slot whenthis was cut. This pressure equals the normal stress in certain areasparallel to the plane of the slot.

In the flat-jack method such as it has been applied, the square orrectangular slot is obtained by means of a series of holes, for instancecm. in diameter each, side-by-side and partly superposed.

In the technique developed at Laboratorio Nacional de Engenharia Civilthe slot is segment-shaped and is cut by means of a disk. This procedurehas the following advantages: 1) the disturbance in the ground isnegligible: 1]) the slot has a sharply defined shape and is very narrowapproximately 5mm.; ill) the time required for the operation isconsiderably reduced.

As for the disturbance in the ground it is obviously of great interestto reduce it as much as possible, due to its possibly great influence onthe state of stress to be determined, especially because the tests areusually carried out at the surface of the ground already much disturbedby the tunnel-driving operations. Regarding the dimensions of the slot,the narrower it is the nearer the basic assumption of the flat-jackmethod is approached; according to this assumption the effect of cuttingthe slot can be cancelled by a normal stress applied on the major facesof the slot i.e. the effect of the stresses on a face normal to themajor faces can be neglected, due to this face being very low. The slotfaces obtained by the cutting disk being very smooth, the cancellingpressure can be directly ap plied on the soil or other medium mass bymeans of flat-jacks that exactly fit the walls without need of mortar orother infilling material, so that the applied pressure is transmittedbetter then in the methods so far used and additionally the timerequired for the infilling material to harden is spared. Otheradvantages are that the flatjacks are reusable and, given the very smallslot height required, the jacks even for high pressures are notdifficult to manufacture. Tests carried out by this procedure developedat the Laboratorio Naeional de Engenharia Civil are very rapid andinexpensive.

The method has the additional advantage of being adequate to determiningstresses at any depth of the slot, for which it suffices to stop thecutting operation and to apply a flat-jack with the dimensions requiredfor the depth of the slot. This operation is facilitated by a set ofeasily movable flat-jacks with approximately the same shape, in whichonly the length of the chord is variable. It is thus possible to detectchanges in the state of stress in the neighborhood of the soil surface.By means of simplifying assumptions on the behavior of the soil or, inthe still more important case ofa rock mass, of the rock, it is possibleto derive the modulus of elasticity of the medium from the curve ofdeformations as a function of the depth of the slot. This is anotherconsiderable advantage of the method.

The equipment which was especially developed to apply this newprocedure; consists of a cutting machine" and flatjacks.

With the above and other objects in view which will become apparent fromthe detailed description below, a preferred form of the invention isshown in the drawings in which:

FlG. la is a side view of the cutting machine,

FIG. 1b is an end elevational view of the machine, and

FIG. 2 is an elevational view of a flat-jack.

The cutting machine as shown in FIGS. 1a and lb is fitted with adiamond-edged disk 1 with a suitable diameter, which is operated by handby means ofthe handles (2) and is guided by four columns (3). A lever(4) facilitates the advance of the disk by making possible its alternatemovement in its own plane, normal to its advance. The disk can beoperated by an electric, compressed-air or other engine, through asliding coupling and two cardan joints (6) which enable the disk to moveand facilitates the assembly of the cutting machine and the engine. Thecutting machine can be bolted (7) on the surface where the slot is goingto be cut. A steel flap (8) protects the operator against projections ofthe water that cools the cutting edge of the disk.

The flat-jacks (FIG. 2), all with the same shape, are very easilyobtained by bending a steel sheet and welding the curved edge 11. Thecell thus formed is filled with a fluid under pressure (oil, water,etc.). The flat-jacks are about as thick or deep as the slot is high. Aflap 23 protects the feeding nipple 22 so as to prevent projections offluid under pressure in case of a rupture in the jack. As indicated theflat-jacks are reusable without difficulty.

A test for determining a stress (including the installation of thecutting machine, the cutting of the slot, the application of theflat-jack and the determination of the cancelling pressure) takes about2 hours even in considerably hard rock or other medium. It is to beunderstood that the improved method can be used in various types of testmedia, whether of relatively soft earth or of relatively hard rock orother more solid type formations, and the use of the term test soilmedium is intended to cover all types.

We claim:

1. A method for more economically, quickly and more accurately obtainingstress determinations existing originally'as in an undisturbed test soilmedium comprising the steps of:

a. disc cutting a smooth, very narrow or thin slot in the surface of themedium to be tested, thereby not unduly disturbing the stabilizednatural structure of the adjoining medium or altering its specificcharacteristics;

b. inserting a flat-type jack of size and thinness correspondingessentially to the slot dimension so as to substantially completely fillthe'slot without need of infilling between the jack sides and testmedium, thereby minimally and negligibly disturbing the state of stressin the test medium;

c. applying on the walls of major dimension of the slot by means offluid pressure directed into said flat-jack, a force to cancel thoseminimal stresses in the test medium resulting from the initial cuttingof the slot, and

d. then measuring the pressure strains exerted by the fluid pressure insaid jack on the sidewalls of the slot whereby initial stress whichexisted in the test medium before the application of a disturbing actionthereto can be readily calculated.

2. A method as defined in claim I further including the step of plottinga curve of deformations as a function of the depth of the slot to thencederive the modulus of elasticity of the test medium.

1. A method for more economically, quickly and more accurately obtainingstress determinations existing originally as in an undisturbed test soilmedium comprising the steps of: a. disc cutting a smooth, very narrow orthin slot in the surface of the medium to be tested, thereby not undulydisturbing the stabilized natural structure of the adjoining medium oraltering its specific characteristics; b. inserting a flat-type jack ofsize and thinness corresponding essentially to the slot dimension so asto substantially completely fill the slot without need of infillingbetween the jack sides and test medium, thereby minimally and negligiblydisturbing the state of stress in the test medium; c. applying on thewalls of major dimension of the slot by means of fluid pressure directedinto said flat-jack, a force to cancel those minimal stresses in thetest medium resulting from the initial cutting of the slot, and d. thenmeasuring the pressure strains exerted by the fluid pressure in saidjack on the sidewalls of the slot whereby initial stress which existedin the test medium before the application of a disturbing action theretocan be readily calculated.
 2. A method as defined in claim 1 furtherincluding the step of plotting a curve of deformations as a function ofthe depth of the slot to thence derive the modulus of elasticity of thetest medium.